1. Field of the Invention
This invention relates to an electronic fuel injecting method and device for an internal combustion engine. More particularly, this invention relates to a D-J type electronic fuel injection system, in which fuel is injected in synchronism with the rotation of the engine unless the operating conditions of the engine reach a predetermined condition, in which case fuel is injected at a predetermined injection time which is not in synchronism with the rotation of the engine.
2. Description of the Prior Art
In electronic fuel injection systems, one injector for each cylinder of the engine or one injector for all of the cylinders may be provided, for example, on an intake manifold or a throttle body of the engine. The valve-opening time period of the injectors or injector is controlled in accordance with the operating conditions of the engine, so that a mixture of a predetermined air-fuel ratio can be supplied to the combustion chambers of the engine. Electronic fuel injection systems are broadly divided into two classes including a so-called L-J type electronic fuel injection system wherein a basic injection time is obtained in accordance with an intake air flowrate of the engine and engine rotational speed and a so-called D-J type electronic fuel injection system wherein a basic injection time is obtained in accordance with an intake pressure of the engine and engine rotational speed.
In the L-J systems, fuel is injected at a constant crank angle in synchronism with the rotation of the engine. In response to operating conditions of the engine, a correction during start, a correction after the start, a correction due to intake air temperature, a correction for warming up, a correction for acceleration during warming up, a correction for power, an air-fuel ratio feedback correction and the like are employed to modify a basic injection time calculated in accordance with an intake air flowrate of the engine rotational speed. When the operating conditions of the engine reach a predetermined condition, fuel is injected at a time not in synchronism with the rotation of the engine, separately of the normal synchronous injection, in order to improve starting performance or responsiveness immediately after acceleration or deceleration. This non-synchronous injection is controlled independently of the synchronous injection. For example, injection may be effected twice when a signal of an ignition switch is detected in order to improve the starting performance of the engine. Injection may be effected one time when a full closed signal of a throttle valve is changed from "ON" to "OFF" to improve engine response and exhaust gas purification performance in moving from the engine idling condition. Injection may be effected one time each time an acceleration signal is generated to improve engine response immediately after acceleration and during acceleration. Finally, injection may be effected one time at the time fuel returns after cut off to improve responsiveness at the time fuel returns.
An L-J type electronic fuel injecting system as described above controls the air-fuel ratio with high accuracy and is widely used in engines with which exhaust gas purification measures are taken. However, heretofore, when requirements for non-synchronous injection take place during synchronous injection, the requirements for non-synchronous injection have been ignored, thus occasionally prohibiting satisfactory performance. To obviate the above-described disadvantage, if the requirements for non-synchronous injection take place during synchronous injection, it may be thought that the time period for synchronous injection should be increased by the time period for non-synchronous injection. In this case, however, if a large number of non-synchronous injections would have taken place beyond the necessary number due to a malfunction such as noise, the fuel injection time would become excessive, whereby the air-fuel ratio would become over-rich, thus possibly causing troubles.
In L-J type electronic fuel injection systems, the dynamic range of the intake air flowrate is so wide that the intake air flowrate during high loads is increased to about 50 times that during idling, thereby presenting the following disadvantages. Namely, not only does the accuracy decrease when the intake air flowrate is converted into a digital signal, but also the bit length of the digital lengthens when the counting accuracy in a digital control circuit is improved after conversion, whereby an expensive computer is required for the digital control circuit. Moreover, in order to measure the intake air flowrate, it is necessary to use a precision measuring instrument, thus presenting the disadvantages of requiring a high installation cost.
On the other hand, with D-J type electronic fuel injection systems, the dynamic range of the intake pressure is so narrow that the variation in the intake pressure is as low as two to three times, so that, not only is the operation in the digital control circuit facilitated, but also, a pressure sensor for detecting intake pressure is inexpensive. However, as compared with the L-J type electronic fuel injection system, the D-J type electronic fuel injection system has a low control accuracy of the air-fuel ratio, and consequently, it has been considered difficult to adopt the D-J type electronic fuel injecting devices into engines employed with exhaust gas purification measures, requiring high accuracy airfuel ratio control. It is also conceivable that, in D-J type electronic fuel injection systems, non-synchronous injection, which has been adopted in L-J type electronic fuel injection systems, may be used. However, the same disadvantages encountered with the L-J type electronic fuel injection system exist.